1. Field of the Invention
The invention lies in the field of communications. More specifically, the present invention relates in general to digital communications systems, in particular to mobile radio systems. Yet more specifically, the invention relates in particular to a transmission and reception method and to a receiver in a digital telecommunications system.
The EDGE (Enhanced Data Rates for GSM Evolution) and the associated packet service EGPRS (Enhanced GPRS) have been defined as a transitional standard between the GSM/GPRS and UMTS mobile radio standards. Both GMSK modulation and 8-PSK modulation are used in the EDGE standard. GMSK modulation uses a signal area with the signal points +1 and −1, while 8-PSK modulation uses a signal area with eight signal points. If the modulation type to be used for the information transmission between a transmitter and a receiver is not predetermined in a fixed manner, then the receiver must be informed of the modulation type which is being used.
Published international PCT application WO 00/10301 and its counterpart U.S. Pat. No. 6,473,506 B1, which are hereby incorporated by reference, describe a method for transmission and identification of the modulation type in digital communication systems. The process makes use of the training sequence which is included in a data burst. Each data burst comprises a fixed predetermined training sequence comprising a sequence of data symbols which is known to the receiver and which is used for channel estimation purposes in the receiver. The prior publication proposes that every feasible modulation type be identified by a specific phase rotation factor, and that the training sequence have a phase rotation factor applied to it, that is to say that the data symbols contained in it be rotated using the phase rotation factor. The same phase rotation factor as that which is also used for the modulation of the payload data is preferably used in that case. As is known, GMSK and 8-PSK modulation are distinguished by using different symbol rotation. While GMSK modulation rotates each transmission symbol onwards through 90 degrees, 8-PSK modulation carries out a rotation of 67.5 degrees per transmission symbol. The data symbols in the transmitted training sequence which have been rotated in phase in this way can be used in the receiver in such a way that, at the start of each data burst, the received training sequence in a number of data paths is rotated back through a corresponding number of phase rotation factors. In the quoted example of the two modulation types GMSK and 8-PSK modulation, the received training sequence is thus rotated back through 90 degrees and 67.5 degrees, respectively. After this, the received and derotated training sequence is compared with a training sequence to which a channel filter function obtained from a channel estimation process has been applied. The comparison is carried out by subtraction of these training sequences from one another, by addition of the squares of the differences, and by detection of the minimum.
This method has the disadvantage that a channel estimation process must be carried out in each of the data paths which are provided for derotation using the various phase rotation factors, and the comparison with the received and respectively derotated training sequence cannot be carried out until the completion of the channel estimation process and the application to the original training sequence of the channel parameters by means of the channel filter function which has been mentioned. The method is very complex and requires a large number of circuit units in the receiver.